Administering thyroid hormone - thyroxin, in its synthetic form - is legitimate for sufferers of hypothyroidism, a condition which causes stunted growth and retarded mental development in children.

Hypothyroidism can be caused by inadequate intake of iodine by women during pregnancy.

In adults, it is more common in women, causing a decrease in metabolic rate, tiredness and lethargy. The condition can also cause menstrual problems.

Older children suffer less severe mental and developmental retardation and mild symptoms of the adult form.

'No justification'

Professor Pat Kendall-Taylor, president of the British Thyroid Association, told BBC News Online: "Thyroid hormone should not be given to patients who do not have clear evidence of thyroid hormone deficiency. There is no justification for it - it should not be done.

"We are very worried about it because we know full well its dangerous effect."

She said a "small number of doctors" were giving the treatment inappropriately.

Dr Daggett said that using the treatment when patients are biochemically normal could be "hazardous".

He and Dr Terry West at the Princess Royal Hospital, Telford, told the British Endocrine Societies Meeting in Birmingham that they have seen 11 patients given thyroid hormones who had no evidence of thyroid deficiency.

Most of the patients consequently showed the side-effects of having excess thyroid hormones in their bodies.

This can lead to hyperthyroidism, which increases the body's metabolic rate and leads to nervousness, sweating, heat intolerance and heart palpitations.

A separate study by a team from Stobhill Hospital, Glasgow, presented at the Birmingham meeting, found that thyroid hormone replacement therapy was no better than a placebo for patients with normal thyroid levels.